Method and apparatus for treating irregular ventricular contractions such as during atrial arrhythmia

ABSTRACT

A cardiac rhythm management system is capable of treating irregular ventricular heart contractions, such as during atrial tachyarrhythmias such as atrial fibrillation. A first indicated pacing interval is computed based at least partially on a most recent V-V interval duration between ventricular beats and a previous value of the first indicated pacing interval. Pacing therapy is provided based on either the first indicated pacing interval or also based on a second indicated pacing interval, such as a sensor-indicated pacing interval. A weighted averager such as an infinite impulse response (IIR) filter adjusts the first indicated pacing interval for sensed beats and differently adjusts the first indicated pacing interval for paced beats. The system regularizes ventricular rhythms by pacing the ventricle, but inhibits pacing when the ventricular rhythms are stable.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.09/316,515, filed on May 21, 1999, the specification of which isincorporated herein by reference.

This application is related to the following commonly assigned patentapplications: “Cardiac Rhythm Management System Promoting AtrialPacing,” U.S. patent application Ser. No. 09/316,682, filed on May 21,1999, now issued as U.S. Pat. No. 6,351,669; “Cardiac Rhythm ManagementSystem With Atrial Shock Timing Optimization,” U.S. patent applicationSer. No. 09/316,741, filed on May 21, 1999, now issued as U.S. Pat. No.6,430,438; and “System Providing Ventricular Pacing and BiventricularCoordination,” U.S. patent application Ser. No. 09/316,588, filed on May21, 1999, now issued as U.S. Pat. No. 6,285,907; the disclosures ofwhich are incorporated herein by reference.

TECHNICAL FIELD

The present system relates generally to cardiac rhythm managementsystems and particularly, but not by way of limitation, to a method andapparatus for treating irregular ventricular contractions, such asduring an atrial arrhythmia.

BACKGROUND

When functioning properly, the human heart maintains its own intrinsicrhythm, and is capable of pumping adequate blood throughout the body'scirculatory system. However, some people have irregular cardiac rhythms,referred to as cardiac arrhythmias. Such arrhythmias result indiminished blood circulation. One mode of treating cardiac arrhythmiasuses drug therapy. Drugs are often effective at restoring normal heartrhythms. However, drug therapy is not always effective for treatingarrhythmias of certain patients. For such patients, an alternative modeof treatment is needed. One such alternative mode of treatment includesthe use of a cardiac rhythm management system. Such systems are oftenimplanted in the patient and deliver therapy to the heart.

Cardiac rhythm management systems include, among other things,pacemakers, also referred to as pacers. Pacers deliver timed sequencesof low energy electrical stimuli, called pace pulses, to the heart, suchas via an intravascular leadwire or catheter (referred to as a “lead”)having one or more electrodes disposed in or about the heart. Heartcontractions are initiated in response to such pace pulses (this isreferred to as “capturing” the heart). By properly timing the deliveryof pace pulses, the heart can be induced to contract in proper rhythm,greatly improving its efficiency as a pump. Pacers are often used totreat patients with bradyarrhythmias, that is, hearts that beat tooslowly, or irregularly.

Cardiac rhythm management systems also include cardioverters ordefibrillators that are capable of delivering higher energy electricalstimuli to the heart. Defibrillators are often used to treat patientswith tachyarrhythmias, that is, hearts that beat too quickly. Suchtoo-fast heart rhythms also cause diminished blood circulation becausethe heart isn't allowed sufficient time to fill with blood beforecontracting to expel the blood. Such pumping by the heart isinefficient. A defibrillator is capable of delivering a high energyelectrical stimulus that is sometimes referred to as a defibrillationcountershock. The countershock interrupts the tachyarrhythmia, allowingthe heart to reestablish a normal rhythm for the efficient pumping ofblood. In addition to pacers, cardiac rhythm management systems alsoinclude, among other things, pacer/defibrillators that combine thefunctions of pacers and defibrillators, drug delivery devices, and anyother implantable or external systems or devices for diagnosing ortreating cardiac arrhythmias.

One problem faced by cardiac rhythm management systems is the propertreatment of ventricular arrhythmias that are caused by atrialtachyarrhythmias such as atrial fibrillation. Atrial fibrillation is acommon cardiac arrhythmia which reduces the pumping efficiency of theheart, though not to as great a degree as in ventricular fibrillation.However, this reduced pumping efficiency requires the ventricle to workharder, which is particularly undesirable in sick patients that cannottolerate additional stresses. As a result of atrial fibrillation,patients may be required to limit their activity and exercise.

Although atrial fibrillation, by itself, is usually notlife-threatening, prolonged atrial fibrillation may be associated withstrokes, which are thought to be caused by blood clots forming in areasof stagnant blood flow. Treating such blood clots requires the use ofanticoagulants. Atrial fibrillation may also cause pain, dizziness, andother irritation to the patient.

An even more serious problem, however, is that atrial fibrillation mayinduce irregular ventricular heart rhythms by processes that are yet tobe fully understood. Such induced ventricular arrhythmias compromisepumping efficiency even more drastically than atrial arrhythmias. Forthese and other reasons, there is a need for a method and apparatus fortreating irregular ventricular contractions during atrial arrhythmiassuch as atrial fibrillation.

SUMMARY OF THE INVENTION

The present system provides a method and apparatus for treatingirregular ventricular contractions, such as during atrial arrhythmias(e.g., atrial fibrillation), or otherwise. The present system providesmany advantages. Among other things, it is capable of treating irregularventricular heart contractions, such as during atrial tachyarrhythmias.It provides a first indicated pacing rate that increases for sensedventricular beats and decreases for paced ventricular beats. The systemdelivers more pacing during irregular sensed beats (such as duringatrial tachyarrhythmias including atrial fibrillation or the like) andless pacing when sensed beats are regular. In a stable heart, the firstindicated pacing rate is typically less than the intrinsic heart rate.This avoids unnecessary pacing of the heart when heart rhythms aresubstantially stable, allowing the heart to beat normally and at its ownintrinsic heart rate.

One aspect of the system permits it to avoid rapid changes in heartrate, and to keep heart rate within acceptable upper and lower limits.The system allows the rate to become more regular and to approach astable rhythm. This provides improved comfort of the patientexperiencing irregular ventricular contractions. In a furtherembodiment, the system includes a second indicated pacing rate, such asa sensor-indicated rate, and provides pacing therapy based on both thefirst and second indicated pacing rates. Other aspects of the inventionwill be apparent on reading the following detailed description of theinvention and viewing the drawings that form a part thereof.

In one embodiment, the system obtains V-V intervals between ventricularbeats. A first indicated pacing interval is computed based at leastpartially on a most recent V-V interval duration and a previous value ofthe first indicated pacing interval. Pacing therapy is provided based onthe first indicated pacing interval.

In a further embodiment, the first indicated pacing interval is adjustedby an amount based at least on the most recent V-V interval duration andthe previous value of the first indicated pacing interval, if the mostrecent V-V interval is concluded by an intrinsic beat. If, however, themost recent V-V interval is concluded by a paced beat, then the firstindicated pacing interval is increased by an amount based at least onthe most recent V-V interval duration and the previous value of thefirst indicated pacing interval.

In another embodiment, the system detects an atrial tachyarrhythmia. Thesystem obtains V-V intervals between ventricular beats. A firstindicated pacing interval is computed based at least partially on a mostrecent V-V interval duration and a previous value of the first indicatedpacing interval. Pacing therapy is provided based on the first indicatedpacing interval, if the atrial tachyarrhythmia is present.

One embodiment provides a cardiac rhythm management system thatincludes, among other things, a ventricular sensing circuit, acontroller, and a ventricular therapy circuit. The controller includes aV-V interval timer, a first register, for storing information associatedwith a first indicated pacing interval, and a filter that updates thefirst indicated pacing interval based on the V-V interval timer and theinformation stored in the first register. Other aspects of the inventionwill be apparent on reading the following detailed description of theinvention and viewing the drawings that form a part thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings, like numerals describe substantially similar componentsthroughout the several views. Like numerals having different lettersuffixes represent different instances of substantially similarcomponents.

FIG. 1 is a schematic drawing illustrating generally one embodiment ofportions of a cardiac rhythm management system and an environment inwhich it is used.

FIG. 2 is a schematic drawing illustrating one embodiment of a cardiacrhythm management device coupled by leads to a heart.

FIG. 3 is a schematic diagram illustrating generally one embodiment ofportions of a cardiac rhythm management device coupled to a heart.

FIG. 4 is a schematic diagram illustrating generally one embodiment of acontroller.

FIG. 5 is a schematic diagram illustrating generally oneconceptualization of portions of a controller.

FIG. 6 is a signal flow diagram illustrating generally one conceptualembodiment of operating a filter.

FIG. 7 is a signal flow diagram illustrating generally anotherconceptualization of operating the filter.

FIG. 8 is a signal flow diagram illustrating generally a furtherconceptualization of operating the filter.

FIG. 9 is a schematic diagram illustrating generally anotherconceptualization of portions of a controller.

FIG. 10 is a schematic diagram illustrating generally a furtherconceptualization of portions of a controller.

FIG. 11 is a graph illustrating generally one embodiment of operating afilter to provide a first indicated pacing rate, such as a VRR indicatedrate, for successive ventricular heart beats.

FIG. 12 is a graph illustrating generally another embodiment ofoperating a filter to provide the first indicated pacing rate, such as aVRR indicated rate, and delivering therapy based on the first indicatedpacing rate and based on a second indicated pacing rate, such as asensor indicated rate.

FIG. 13 is a graph illustrating generally another illustrative exampleof heart rate vs. time according to a VRR algorithm spreadsheetsimulation.

FIG. 14 is a graph illustrating generally one embodiment of using atleast one of coefficients a and b as a function of heart rate (or acorresponding time interval).

DETAILED DESCRIPTION

In the following detailed description, reference is made to theaccompanying drawings which form a part hereof, and in which is shown byway of illustration specific embodiments in which the invention may bepracticed. These embodiments are described in sufficient detail toenable those skilled in the art to practice the invention, and it is tobe understood that the embodiments may be combined, or that otherembodiments may be utilized and that structural, logical and electricalchanges may be made without departing from the spirit and scope of thepresent invention. The following detailed description is, therefore, notto be taken in a limiting sense, and the scope of the present inventionis defined by the appended claims and their equivalents. In thedrawings, like numerals describe substantially similar componentsthroughout the several views. Like numerals having different lettersuffixes represent different instances of substantially similarcomponents.

The present methods and apparatus will be described in applicationsinvolving implantable medical devices including, but not limited to,implantable cardiac rhythm management systems such as pacemakers,cardioverter/defibrillators, and pacer/defibrillators. However, it isunderstood that the present methods and apparatus may be employed inunimplanted devices, including, but not limited to, external pacemakers,cardioverter/defibrillators, pacer/defibrillators, monitors, programmersand recorders.

Problems Associated With Atrial Arrhythmias

As stated earlier, one potential cause of irregularity of ventricularcontractions arises during atrial tachyarrhythmias, such as atrialfibrillation. During atrial fibrillation, irregular ventricularcontractions may be caused by a conducted atrial tachyarrhythmia, thenpacing the ventricle will regularize the ventricular heart rate byestablishing retrograde conduction from the ventricle. This, in turn, isbelieved to block forward conduction of atrial signals through theatrioventricular (A-V) node. As a result, irregular atrial signals donot trigger resulting irregular ventricular contractions.

One therapy for treating irregular ventricular contractions duringatrial fibrillation is to increase the ventricular heart rate by pacingthe ventricle at a higher rate than the unpaced (intrinsic) ventricularheart rate. Such therapy is believed to decrease the discomfortexperienced by the patient having atrial arrhythmia because it regulatesthe ventricular contractions to avoid short periods between contractionsand/or long periods without a contraction. Such therapy is also believedto decrease the ability of the atrial fibrillation to induce irregularventricular contractions.

An increase in rate of ventricular contractions, however, must be donecarefully to avoid pacing the heart at an unnecessarily high rate.Furthermore, such a therapy should not impose pacing where normal or“intrinsic” heart pacing is adequate such as, for example, when atrialtachyarrhythmias no longer cause disorder of ventricular contractions.As long as the heart is actively paced, it may be difficult orimpossible to determine when to cease such a therapy. One advantage ofthe present system is that it allows intrinsic ventricular rhythms, ifsuch rhythms are regular, but provides pacing that stabilizesventricular rhythms if they become irregular, as discussed below.

General System Overview and Examples

This document describes, among other things, a cardiac rhythm managementsystem providing a method and apparatus for treating irregularventricular contractions during atrial arrhythmia. FIG. 1 is a schematicdrawing illustrating, by way of example, but not by way of limitation,one embodiment of portions of a cardiac rhythm management system 100 andan environment in which it is used. In FIG. 1, system 100 includes animplantable cardiac rhythm management device 105, also referred to as anelectronics unit, which is coupled by an intravascular endocardial lead110, or other lead, to a heart 115 of patient 120. System 100 alsoincludes an external programmer 125 providing wireless communicationwith device 105 using a telemetry device 130. Catheter lead 110 includesa proximal end 135, which is coupled to device 105, and a distal end140, which is coupled to one or more portions of heart 115.

FIG. 2 is a schematic drawing illustrating, by way of example, but notby way of limitation, one embodiment of device 105 coupled by leads110A-B to heart 115, which includes a right atrium 200A, a left atrium200B, a right ventricle 205A, a left ventricle 205B, and a coronarysinus 220 extending from right atrium 200A. In this embodiment, atriallead 110A includes electrodes (electrical contacts) disposed in, around,or near an atrium 200 of heart 115, such as ring electrode 225 and tipelectrode 230, for sensing signals and/or delivering pacing therapy tothe atrium 200. Lead 110A optionally also includes additionalelectrodes, such as for delivering atrial and/or ventricularcardioversion/defibrillation and/or pacing therapy to heart 115.

In FIG. 2, a ventricular lead 110B includes one or more electrodes, suchas tip electrode 235 and ring electrode 240, for delivering sensingsignals and/or delivering pacing therapy. Lead 110B optionally alsoincludes additional electrodes, such as for delivering atrial and/orventricular cardioversion/defibrillation and/or pacing therapy to heart115. Device 105 includes components that are enclosed in ahermetically-sealed can 250. Additional electrodes may be located on thecan 250, or on an insulating header 255, or on other portions of device105, for providing unipolar pacing and/or defibrillation energy inconjunction with the electrodes disposed on or around heart 115. Otherforms of electrodes include meshes and patches which may be applied toportions of heart 115 or which may be implanted in other areas of thebody to help “steer” electrical currents produced by device 105. Thepresent method and apparatus will work in a variety of configurationsand with a variety of electrical contacts or “electrodes.”

EXAMPLE CARDIAC RHYTHM MANAGEMENT DEVICE

FIG. 3 is a schematic diagram illustrating generally, by way of example,but not by way of limitation, one embodiment of portions of device 105,which is coupled to heart 115. Device 105 includes a power source 300,an atrial sensing circuit 305, a ventricular sensing circuit 310, aventricular therapy circuit 320, and a controller 325.

Atrial sensing circuit 305 is coupled by atrial lead 110A to heart 115for receiving, sensing, and/or detecting electrical atrial heartsignals. Such atrial heart signals include atrial activations (alsoreferred to as atrial depolarizations or P-waves), which correspond toatrial contractions. Such atrial heart signals include normal atrialrhythms, and abnormal atrial rhythms including atrial tachyarrhythmias,such as atrial fibrillation, and other atrial activity. Atrial sensingcircuit 305 provides one or more signals to controller 325, via node/bus327, based on the received atrial heart signals. Such signals providedto controller 325 indicate, among other things, the presence of atrialfibrillation.

Ventricular sensing circuit 310 is coupled by ventricular lead 10B toheart 115 for receiving, sensing, and/or detecting electricalventricular heart signals, such as ventricular activations (alsoreferred to as ventricular depolarizations or R-waves), which correspondto ventricular contractions. Such ventricular heart signals includenormal ventricular rhythms, and abnormal ventricular rhythms, includingventricular tachyarrhythmias, such as ventricular fibrillation, andother ventricular activity, such as irregular ventricular contractionsresulting from conducted signals from atrial fibrillation. Ventricularsensing circuit 310 provides one or more signals to controller 325, vianode/bus 327, based on the received ventricular heart signals. Suchsignals provided to controller 325 indicate, among other things, thepresence of ventricular depolarizations, whether regular or irregular inrhythm.

Ventricular therapy circuit 320 provides ventricular pacing therapy, asappropriate, to electrodes located at or near one of the ventricles 205of heart 115 for obtaining resulting evoked ventricular depolarizations.In one embodiment, ventricular therapy circuit 320 also providescardioversion/defibrillation therapy, as appropriate, to electrodeslocated at or near one of the ventricles 205 of heart 115, forterminating ventricular fibrillation and/or other ventriculartachyarrhythmias.

Controller 325 controls the delivery of therapy by ventricular therapycircuit 320 and/or other circuits, based on heart activity signalsreceived from atrial sensing circuit 305 and ventricular sensing circuit310, as discussed below. Controller 325 includes various modules, whichare implemented either in hardware or as one or more sequences of stepscarried out on a microprocessor or other controller. Such modules areillustrated separately for conceptual clarity; it is understood that thevarious modules of controller 325 need not be separately embodied, butmay be combined and/or otherwise implemented, such as insoftware/firmware.

In general terms, sensing circuits 305 and 310 sense electrical signalsfrom heart tissue in contact with the catheter leads 110A-B to whichthese sensing circuits 305 and 310 are coupled. Sensing circuits 305 and310 and/or controller 325 process these sensed signals. Based on thesesensed signals, controller 325 issues control signals to therapycircuits, such as ventricular therapy circuit 320, if necessary, for thedelivery of electrical energy (e.g., pacing and/or defibrillationpulses) to the appropriate electrodes of leads 110A-B. Controller 325may include a microprocessor or other controller for execution ofsoftware and/or firmware instructions. The software of controller 325may be modified (e.g., by remote external programmer 105) to providedifferent parameters, modes, and/or functions for the implantable device105 or to adapt or improve performance of device 105.

In one further embodiment, one or more sensors, such as sensor 330, mayserve as inputs to controller 325 for adjusting the rate at which pacingor other therapy is delivered to heart 115. One such sensor 330 includesan accelerometer that provides an input to controller 325 indicatingincreases and decreases in physical activity, for which controller 325increases and decreases pacing rate, respectively. Another such sensorincludes an impedance measurement, obtained from body electrodes, whichprovides an indication of increases and decreases in the patient'srespiration, for example, for which controller 325 increases anddecreases pacing rate, respectively. Any other sensor 330 providing anindicated pacing rate can be used.

FIG. 4 is a schematic diagram illustrating generally, by way of example,but not by way of limitation, one embodiment of controller 325 thatincludes several different inputs to modify the rate at which pacing orother therapy is delivered. For example, Input #1 may provideinformation about left ventricular rate, Input #2 may provide anaccelerometer-based indication of activity, and Input #3 may provide animpedance-based indication of respiration, such as minute ventilation.Based on at least one of these and/or other inputs, controller 325provides an output indication of pacing rate as a control signaldelivered to a therapy circuit, such as to ventricular therapy circuit320. Ventricular therapy circuit 320 issues pacing pulses based on oneor more such control signals received from controller 325. Control ofthe pacing rate may be performed by controller 325, either alone or incombination with peripheral circuits or modules, using software,hardware, firmware, or any combination of the like. The softwareembodiments provide flexibility in how inputs are processed and may alsoprovide the opportunity to remotely upgrade the device software whilestill implanted in the patient without having to perform surgery toremove and/or replace the device 105.

CONTROLLER EXAMPLE 1

FIG. 5 is a schematic diagram illustrating generally, by way of example,but not by way of limitation, one conceptualization of portions ofcontroller 325. At least one signal from ventricular sensing circuit 310is received by ventricular event module 500, which recognizes theoccurrence of ventricular events included within the signal. Such eventsare also referred to as “beats,” “activations,” “depolarizations,” “QRScomplexes,” “R-waves,” “contractions.” Ventricular event module 500detects intrinsic events (also referred to as sensed events) from thesignal obtained from ventricular sensing circuit 310. Ventricular eventmodule 500 also detects evoked events (resulting from a pace) eitherfrom the signal obtained from ventricular sensing circuit 310, orpreferably from a ventricular pacing control signal obtained from pacingcontrol module 505, which also triggers the delivery of a pacingstimulus by ventricular therapy circuit 320. Thus, ventricular eventsinclude both intrinsic/sensed events and evoked/paced events.

A time interval between successive ventricular events, referred to as aV-V interval, is recorded by a first timer, such as V-V interval timer510. A filter 515 computes a “first indicated pacing interval,” i.e.,one indication of a desired time interval between ventricular events or,stated differently, a desired ventricular heart rate. The firstindicated pacing interval is also referred to as a ventricular rateregularization (VRR) indicated pacing interval. In various embodiments,filter 515 includes an averager, a weighted averager, a median filter,an infinite (IIR) filter, a finite impulse response (FIR) filter, or anyother analog or digital signal processing circuit providing the desiredsignal processing described more particularly below.

In one embodiment, filter 515 computes a new value of the firstindicated pacing interval based on the duration of the most recent V-Vinterval recorded by timer 510 and on a previous value of the firstindicated pacing interval stored in first indicated pacing intervalregister 520. Register 520 is then updated by storing the newly computedfirst indicated pacing interval in register 520. Based on the firstindicated pacing interval stored in register 520, pacing control module505 delivers control signals to ventricular therapy circuit 320 fordelivering therapy, such as pacing stimuli, at the VRR-indicatedventricular heart rate corresponding to the inverse of the duration ofthe first indicated pacing interval.

FILTER EXAMPLE 1

In general terms, for one embodiment, device 105 obtains V-V intervalsbetween successive sensed or evoked ventricular beats. Device 105computes a new first indicated pacing interval based at least in part onthe duration of the most recent V-V interval and a previous value of thefirst indicated pacing interval. Device 105 provides pacing therapydelivered at a rate corresponding to the inverse of the duration of thefirst indicated pacing interval.

FIG. 6 is a signal flow diagram illustrating generally, by way ofexample, but not by way of limitation, one embodiment of operatingfilter 515. Upon the occurrence of a sensed or evoked ventricular beat,timer 510 provides filter 515 with the duration of the V-V intervalconcluded by that beat, which is referred to as the most recent V-Vinterval (VV_(n)). Filter 515 also receives the previous value of thefirst indicated pacing interval (T_(n-1)) stored in register 520. Themost recent V-V interval VV_(n) and the previous value of the firstindicated pacing interval T_(n-1) are each scaled by respectiveconstants A and B, and then summed to obtain a new value of the firstindicated pacing interval (T_(n)), which is stored in register 520 andprovided to pacing control module 505. In one embodiment, thecoefficients A and B are different values, and are either programmable,variable, or constant.

If no ventricular beat is sensed during the new first indicated pacinginterval T_(n), which is measured as the time from the occurrence of theventricular beat concluding the most recent V-V interval VV_(n), thenpacing control module 505 instructs ventricular therapy circuit 320 todeliver a ventricular pacing pulse upon the expiration of the new firstindicated pacing interval T_(n). In one embodiment, operation of thefilter is described by T_(n)=A·VV_(n)+B·T_(n-1), where A and B arecoefficients (also referred to as “weights”), VV_(n) is the most recentV-V interval duration, and T_(n-1) is the previous value of the firstindicated pacing interval.

Initialization of filter 515 includes seeding the filter by storing, inregister 520, an initial interval value. In one embodiment, register 520is initialized to an interval value corresponding to a lower rate limit(LRL), i.e., a minimum rate at which pacing pulses are delivered bydevice 105. Register 520 could alternatively be initialized with anyother suitable value.

FILTER EXAMPLE 2

In one embodiment, operation of filter 515 is based on whether the beatconcluding the most recent V-V interval VV_(n) is a sensed/intrinsicbeat or a paced/evoked beat. In this embodiment, the pacing controlmodule 505, which controls the timing and delivery of pacing pulses,provides an input to filter 515 that indicates whether the most recentV-V interval VV_(n) was concluded by an evoked beat initiated by apacing stimulus delivered by device 105, or was concluded by anintrinsic beat sensed by ventricular sensing circuit 310.

In general terms, if the most recent V-V interval VV_(n) is concluded bya sensed/intrinsic beat, then filter 515 provides a new first indicatedpacing interval T_(n) that is adjusted from the value of the previousfirst indicated pacing interval T_(n-1) such as, for example, decreasedby an amount that is based at least partially on the duration of themost recent V-V interval VV_(n) and on the duration of the previousvalue of the first indicated pacing interval T_(n-1). If, however, themost recent V-V interval VVn is concluded by a paced/evoked beat, thenfilter 515 provides a new first indicated pacing interval T_(n) that isincreased from the value of the previous first indicated pacing intervalT_(n-1), such as, for example, by an amount that is based at leastpartially on the duration of the most recent V-V interval VV_(n) and onthe duration of the previous value of the first indicated pacinginterval T_(n-1). If no ventricular beat is sensed during the new firstindicated pacing interval T_(n), which is measured as the time from theoccurrence of the ventricular beat concluding the most recent V-Vinterval VV_(n), then pacing control module 505 instructs ventriculartherapy circuit 320 to deliver a ventricular pacing pulse upon theexpiration of the new first indicated pacing interval T_(n).

FIG. 7 is a signal flow diagram, illustrating generally, by way ofexample, but not by way of limitation, another conceptualization ofoperating filter 515, with certain differences from FIG. 6 moreparticularly described below. In this embodiment, the pacing controlmodule 505, which controls the timing and delivery of pacing pulses,provides an input to filter 515 that indicates whether the most recentV-V interval VV_(n) was concluded by an evoked beat initiated by apacing stimulus delivered by device 105, or was concluded by anintrinsic beat sensed by ventricular sensing circuit 310.

If the most recent V-V interval VV_(n) was concluded by an intrinsicbeat, then the most recent V-V interval VV_(n) and the previous value ofthe first indicated pacing interval T_(n-1) are each scaled byrespective constants A and B, and then summed to obtain the new value ofthe first indicated pacing interval T_(n), which is stored in register520 and provided to pacing control module 505. Alternatively, if themost recent V-V interval VV_(n) was concluded by a evoked/paced beat,then the most recent V-V interval VV_(n) and the previous value of thefirst indicated pacing interval T_(n-1) are each scaled by respectiveconstants C and D, and then summed to obtain the new value of the firstindicated pacing interval T_(n), which is stored in register 520 andprovided to pacing control module 505. In one embodiment, thecoefficients C and D are different from each other, and are eitherprogrammable, variable, or constant. In a further embodiment, thecoefficient C is a different value from the coefficient A, and/or thecoefficient D is a different value than the coefficient B, and thesecoefficients are either programmable, variable, or constant. In anotherembodiment, the coefficient D is the same value as the coefficient B.

In one embodiment, operation of filter 515 is described byT_(n)=A·VV_(n)+B·T_(n-1), if VV_(n) is concluded by an intrinsic beat,and is described by T_(n)=C·VV_(n)+D·T_(n-1), if VV_(n) is concluded bya paced beat, where A, B, C and D are coefficients (also referred to as“weights”), VV_(n) is the most recent V-V interval duration, T_(n) isthe new value of the first indicated pacing interval, and T_(n-1) is theprevious value of the first indicated pacing interval. If no ventricularbeat is sensed during the new first indicated pacing interval T_(n),which is measured as the time from the occurrence of the ventricularbeat concluding the most recent V-V interval VV_(n), then pacing controlmodule 505 instructs ventricular therapy circuit 320 to deliver aventricular pacing pulse upon the expiration of the new first indicatedpacing interval T_(n).

FILTER EXAMPLE 3

In another embodiment, these coefficients can be more particularlydescribed using an intrinsic coefficient (a), a paced coefficient (b),and a weighting coefficient (w). In one such embodiment, A=a·w, B=(1−w),C=b·w, and D=(1−w). In one example, operation of the filter 515 isdescribed by T_(n)=a·w·VV_(n)+(1−w)·T_(n-1), if VV_(n) is concluded byan intrinsic beat, otherwise is described byT_(n)=b·w·VV_(n)+(1−w)·T_(n-1), if VV_(n) is concluded by a paced beat,as illustrated generally, by way of example, but not by way oflimitation, in the signal flow graph of FIG. 8. If no ventricular beatis sensed during the new first indicated pacing interval T_(n), which ismeasured as the time from the occurrence of the ventricular beatconcluding the most recent V-V interval VV_(n), then pacing controlmodule 505 instructs ventricular therapy circuit 320 to deliver aventricular pacing pulse upon the expiration of the new first indicatedpacing interval T_(n). In one embodiment, the coefficients a and b aredifferent from each other, and are either programmable, variable, orconstant.

The above-described parameters (e.g., A, B, C, D, a, b, w) are stated interms of time intervals (e.g., VV_(n), T_(n), T_(n-1)). However, analternate system may produce results in terms of rate, rather than timeintervals, without departing from the present method and apparatus. Inone embodiment, weighting coefficient w, intrinsic coefficient a, andpaced coefficient b, are variables. Different selections of w, a, and b,will result in different operation of the present method and apparatus.For example, as w increases the weighting effect of the most recent V-Vinterval VV_(n) increases and the weighting effect of the previous firstindicated pacing rate T_(n-1) decreases. In one embodiment, w=1/16=0.0625. In another embodiment, w= 1/32. Another possible range forw is from w=½ to w= 1/1024. A further possible range for w is from w≈0to w≈1. Other values of w, which need not include division by powers oftwo, may be substituted without departing from the present method andapparatus.

In one embodiment, intrinsic coefficient a, is selected to be greaterthan 0.5, or to be greater than 1.0. In one example, the intrinsiccoefficient a is selected to be lesser in value than the pacingcoefficient b. In one example, a≈1.1 and b≈1.2. In another embodimenta=0.9 and b=1.1. One possible range for a is from a=0.5 to a=2.0, andfor b is from b=1.0 to b=3.0. The coefficients may vary withoutdeparting from the present method and apparatus.

In one embodiment, for b>1 and for substantially regular V-V intervals,filter 515 provides a new first indicated pacing interval T_(n) that isat least slightly longer than the expected intrinsic V-V interval beingmeasured by timer 515. Thus, if the intrinsic V-V interval being timedis consistent with the duration of previously received V-V intervals,then filter 515 avoids triggering a pacing stimulus. In such a case, apacing pulse is delivered only if the presently timed V-V intervalbecomes longer than the previous substantially constant V-V intervals.In general terms, filter 515 operates so that pacing pulses aretypically inhibited if the ventricular rate is substantially constant.However, if the measured V-V intervals become irregular, then filter 515operates, over a period of one or several such V-V intervals, to shortenthe first indicated pacing interval T_(n) so that pacing stimuli arebeing delivered.

According to one aspect of the invention, it is believed that if theirregular V-V intervals are caused by a conducted atrialtachyarrhythmia, then pacing the ventricle will regularize theventricular heart rate by establishing retrograde conduction from theventricle. This, in turn, blocks forward conduction of atrial signalsthrough the atrioventricular (A-V) node. As a result, irregular atrialsignals do not trigger resulting irregular ventricular contractions.According to another aspect of the invention, however, this method andapparatus will not introduce pacing pulses until the heartbeat becomesirregular. Therefore, the heart is assured to pace at its intrinsic ratewhen regular ventricular contractions are sensed.

CONTROLLER EXAMPLE 2

FIG. 9 is a schematic diagram illustrating generally, by way of example,but not by way of limitation, another conceptualization of portions ofcontroller 325, with certain differences from FIG. 5 more particularlydescribed below. In FIG. 9, controller 325 receives from sensor 330 asignal including information from which a physiologically desired heartrate (e.g., based on the patient's activity, respiration, or any othersuitable indicator of metabolic need) can be derived. The sensor signalis digitized by an A/D converter 900. The digitized signal is processedby a sensor rate module 905, which computes a desired heart rate that isexpressed in terms of a second indicated pacing interval stored inregister 910.

Pacing control module 505 delivers a control signal, which directsventricular therapy circuit 320 to deliver a pacing pulse, based oneither (or both) of the first or second indicated pacing intervals,stored in registers 520 and 910, respectively, or both. In oneembodiment, pacing control module 505 includes a selection module 915that selects between the new first indicated pacing interval T_(n) andthe sensor-based second indicated pacing interval.

In one embodiment, selection module 915 selects the shorter of the firstand second indicated pacing intervals as the selected indicated pacinginterval S_(n). If no ventricular beat is sensed during the selectedindicated pacing interval S_(n), which is measured as the time from theoccurrence of the ventricular beat concluding the most recent V-Vinterval VV_(n), then pacing control module 505 instructs ventriculartherapy circuit 320 to deliver a ventricular pacing pulse upon theexpiration of the selected indicated pacing interval S_(n).

In general terms, for this embodiment, the ventricle is paced at thehigher of the sensor indicated rate and the VRR indicated rate. If, forexample, the patient is resting, such that the sensor indicated rate islower than the patient's intrinsic rate, and the patient's intrinsicrate is substantially constant, then the intrinsic rate is higher thanthe VRR indicated rate. As a result, pacing pulses generally will not bedelivered. But if, for example, the patient is resting, but with anatrial tachyarrhythmia that induces irregular ventricular contractions,then pacing pulses generally will be delivered at the VRR indicatedrate. In another example, if the patient is active, such that the sensorindicated rate is higher than the VRR indicated rate, then pacing pulsesgenerally will be delivered at the sensor indicated rate. In analternative embodiment, the pacing rate is determined by blending thesensor indicated rate and the VRR indicated rate, rather than byselecting the higher of these two indicated rates (i.e., the shorter ofthe first and second indicated pacing intervals).

In another embodiment, selection module 915 provides a selectedindicated pacing interval S_(n) based on a blending of both the firstand second indicated pacing intervals. In one such example, selectionmodule 915 applies predetermined or other weights to the first andsecond indicated pacing intervals to compute the selected pacinginterval S_(n).

CONTROLLER EXAMPLE 2

FIG. 10 is a schematic diagram illustrating generally, by way ofexample, but not by way of limitation, another conceptualization ofportions of controller 325, with certain differences from FIG. 9 moreparticularly described below. In FIG. 10, controller 325 includes anatrial tachyarrhythmia (AT) detection module 1000 that receives a signalfrom atrial sensing circuit 305. The received signal includesinformation about atrial events, from which AT detection module 1000determines the presence or absence of one or more atrialtachyarrhythmias, such as atrial fibrillation.

In one embodiment, AT detection module 1000 provides a control signal,to pacing control module 505, that indicates the presence or absence ofan atrial tachyarrhythmia, such as atrial fibrillation. In oneembodiment, selection module 915 selects between the first and secondindicated pacing intervals as illustrated, by way of example, but not byway of limitation, in Table 1. TABLE 1 Example Selection Based on ATDetection, 1st Indicated Pacing Interval, and 2nd Indicated PacingInterval 1st Indicated Pacing 1st Indicated Pacing Interval < 2ndIndicated Interval ≧ 2nd Indicated AT Present? Pacing Interval ? PacingInterval ? Yes, AT Present S_(n)

1st Indicated S_(n)

2nd Indicated Pacing Interval Pacing Interval (i.e., VRR) (e.g., Sensor)No, AT not Present S_(n)

2nd Indicated S_(n)

2nd Indicated Pacing Interval Pacing Interval (e.g., Sensor) (e.g.,Sensor)

In this embodiment, if an atrial tachyarrhythmia is present and thefirst indicated pacing interval is shorter than the second indicatedpacing interval, then selection module 915 selects the first indicatedpacing interval, which is based on the VRR techniques described above,as the selected indicated pacing interval S_(n). Otherwise, selectionmodule 915 selects the second indicated pacing interval, which in oneembodiment is based on the sensor indications, as the selected indicatedpacing interval S_(n). As discussed above, if no ventricular beat issensed during the selected indicated pacing interval S_(n), which ismeasured as the time from the occurrence of the ventricular beatconcluding the most recent V-V interval VV_(n), then pacing controlmodule 505 instructs ventricular therapy circuit 320 to deliver aventricular pacing pulse upon the expiration of the selected indicatedpacing interval S_(n).

Stated differently, for this embodiment, the ventricle is paced at theVRR indicated rate only if an atrial tachyarrhythmia, such as atrialfibrillation, is present and the VRR indicated rate exceeds the sensorindicated rate. Otherwise the ventricle is paced at the sensor indicatedrate. If, for example, the patient is resting, such that the sensorindicated rate is lower than the patient's intrinsic rate, and no atrialtachyarrhythmia is present, then the device will sense the intrinsicrate or will deliver ventricular paces at the lower rate limit. But if,for example, the patient is resting, but with an atrial tachyarrhythmiathat induces irregular ventricular contractions, then pacing pulsesgenerally will be delivered at the VRR indicated rate. In anotherexample, if the patient is active, such that the sensor indicated rateis higher than the VRR indicated rate, then pacing pulses generally willbe delivered at the sensor indicated rate, whether or not atrialtachyarrhythmia is present. As an alternative to the selection describedwith respect to Table 1, selection module 915 provides a fixed orvariable weighting or blending of both the sensor-indicated rate and VRRindicated rate, such that pacing pulses are delivered based on theblended rate.

The second indicated pacing interval need not be based on sensorindications. In one embodiment, for example, the second indicated pacinginterval tracks the sensed atrial heart rate when no atrialtachyarrhythmia is present. In this embodiment, selection module 915performs a mode-switching function in which the first indicated pacinginterval is used whenever atrial tachyarrhythmia is present and thesecond indicated pacing interval (e.g., atrial-tracking) is used when noatrial tachyarrhythmia is present.

In another embodiment, heart rate/interval is used as a trigger turnon/off use of the first indicated pacing interval (e.g., the VRRindicated pacing interval). In one example, pacing therapy is based onthe first indicated pacing interval if the first indicated pacinginterval is longer than a first predetermined value, and pacing therapyis substantially independent of the first indicated pacing interval ifthe first indicated pacing interval is shorter than the firstpredetermined value. In this example, the VRR indicated pacing intervalis used at low heart rates, but not at fast heart rates.

FILTER RATE BEHAVIOR EXAMPLE 1

FIG. 11 is a graph illustrating generally, by way of example, but not byway of limitation, one embodiment of a VRR indicated rate for successiveventricular heart beats for one mode of operating filter 515. Asdiscussed above, the VRR indicated rate is simply the frequency, betweenventricular heart beats, associated with the first indicated pacinginterval. Stated differently, the VRR indicated rate is the inverse ofthe duration of the first indicated pacing interval. If pacing is basedsolely on the VRR indicated rate, pacing control module 505 directsventricular therapy circuit 320 to issue a pacing pulse after the timesince the last ventricular beat equals or exceeds the first indicatedpacing interval. However, as described above, in certain embodiments,pacing control module 505 directs ventricular therapy circuit 320 toissue a pacing pulse based on factors other than the VRR indicated ratesuch as for, example, based on the sensor indicated rate.

In the example illustrated in FIG. 11, a first sensed intrinsicventricular beat, indicated by an “S” was detected just beforeexpiration of the first indicated pacing interval (“VRR indicated pacinginterval”) T₀, as computed based on a previous ventricular beat. In oneembodiment, the new VRR indicated pacing interval T₁ is computed basedon the duration of most recent V-V interval VV₁ and a previous value ofthe VRR indicated pacing interval T₀, as discussed above. In thisexample, the new VRR indicated pacing interval T₁ corresponds to a lowerrate limit (LRL) time interval. In one embodiment, the allowable rangeof the VRR indicated pacing interval is limited so that the VRRindicated pacing interval does not exceed the duration of the LRL timeinterval, and so that the VRR indicated pacing interval is not shorterthan the duration of an upper rate limit (URL) time interval.

The second ventricular beat is also sensed, just before expiration ofthe VRR indicated pacing interval T₁. In one embodiment, the new VRRindicated pacing interval T₂ is computed based on the duration of mostrecent V-V interval VV₂ and a previous value of the VRR indicated pacinginterval, T₁, as discussed above. The first and second ventricular beatsrepresent a stable intrinsic rhythm, for which no pacing is deliveredbecause the VRR indicated pacing interval is at a lower rate than thesensed intrinsic ventricular beats.

The third, fourth, and fifth ventricular beats represent the onset ofatrial fibrillation, resulting in erratic ventricular rates. The thirdventricular beat is sensed well before expiration of the VRR indicatedpacing interval T₂, such that no pacing pulse is issued. For the sensedthird ventricular beat, filter 515 computes the new VRR indicated pacinginterval T₃ as being shorter in duration relative to the previous VRRindicated pacing interval T₂.

The fourth ventricular beat is similarly sensed well before expirationof the VRR indicated pacing interval T₃, such that no pacing pulse isissued. For the sensed fourth ventricular beat, filter 515 computes thenew VRR indicated pacing interval T₄ as being shorter in durationrelative to the previous VRR indicated pacing interval T₃.

The fifth ventricular beat is sensed before expiration of the VRRindicated pacing interval T₄, such that no pacing pulse is issued. Forthe sensed fifth ventricular beat, filter 515 computes the new VRRindicated pacing interval T₅ as being shorter in duration relative tothe previous VRR indicated pacing interval T₄.

The sixth, seventh, and eighth ventricular beats indicate regularizationof the ventricular rate using the pacing techniques described above. Noventricular beat is sensed during the VRR indicated pacing interval T₅,so a pacing pulse is issued to evoke the sixth ventricular beat. A newVRR indicated pacing interval T₆ is computed as being increased induration relative to the previous VRR indicated pacing interval T₅,lowering the VRR indicated rate. Similarly, no ventricular beat issensed during the VRR indicated pacing interval.

The ninth ventricular beat represents another erratic ventricular beatresulting from the atrial fibrillation episode. The ninth ventricularbeat is sensed before expiration of the VRR indicated pacing intervalT₈. As a result, a shorter new VRR indicated pacing interval T₉ iscomputed.

The tenth and eleventh ventricular beats illustrate furtherregularization of the ventricular rate using the pacing techniquesdescribed above. No ventricular beat is sensed during the VRR indicatedpacing interval T₉, so a pacing pulse is issued to evoke the tenthventricular beat. A new VRR indicated pacing interval T₁₀ is computed asbeing increased in duration relative to the previous VRR indicatedpacing interval T₉, lowering the VRR indicated rate. Similarly, noventricular beat is sensed during the VRR indicated pacing interval T₁₀,so a pacing pulse is issued to evoke the tenth ventricular beat. A newVRR indicated pacing interval T₁₁ is compute as being increased induration relative to the previous VRR indicated pacing interval T₁₀,lowering the VRR indicated rate.

The twelfth, thirteenth, fourteenth, and fifteenth ventricular beatsillustrate resumption of a stable intrinsic rhythm after termination ofthe atrial fibrillation episode. For such a stable rate, the VRRindicated rate proceeds asymptotically toward a “floor value” thattracks, but remains below, the intrinsic rate. This allows the intrinsicheart signals to control heart rate when such intrinsic heart signalsprovide a stable rhythm. As a result, when the patient's intrinsic rateis constant, paces will be withheld, allowing the patient's intrinsicheart rhythm to continue. If the patient's heart rate includes somevariability, and the VRR indicated floor value is close to the meanintrinsic heart rate, then occasional paced beats will occur. Such pacebeats will gradually lengthen the VRR indicated pacing interval, therebyallowing subsequent intrinsic behavior when the patient's heart ratebecomes substantially constant.

The intrinsic coefficient a of filter 515 controls the “attack slope” ofthe VRR indicated heart rate as the VRR indicated heart rate increasesbecause of sensed intrinsic beats. The paced coefficient b of filter 515controls the “decay slope” of the VRR indicated heart rate as the VRRindicated heart rate decreases during periods of paced beats. In oneembodiment, in which a>1.0 and b>1.0, decreasing the value of a toward1.0 increases the attack slope such that the VRR indicated rateincreases faster in response to sensed intrinsic beats, while decreasingthe value of b toward 1.0 decreases the decay slope such that the VRRindicated rate decreases more slowly during periods of paced beats.Conversely, for a>1.0 and b>1.0, increasing the value of a from 1.0decreases the attack slope such that the VRR indicated rate increasesmore slowly in response to sensed intrinsic beats, while increasing thevalue of b from 1.0 increases the decay slope such that theVRR-indicated rate decreases more quickly during periods of paced beats.

In one embodiment, for a>1.0 and b>1.0, decreasing both a and b toward1.0 increases VRR indicated rate during periods of sensed intrinsicactivity so that the VRR indicated rate is closer to the mean intrinsicrate. Because the VRR indicated rate is closer to the mean intrinsicrate, variability in the intrinsic heart rate is more likely to triggerpaces at the VRR indicated rate. On the other hand, for a>1.0 and b>1.0,increasing both a and b from 1.0 decreases the VRR indicated rate duringperiods of sensed intrinsic activity so that the VRR indicated rate isfarther beneath the mean intrinsic rate. Because the VRR indicated rateis farther beneath the mean intrinsic rate, the same variability in theintrinsic heart rate becomes less likely to trigger paces at the VRRindicated rate.

In one embodiment, these coefficients are programmable by the user, suchas by using remote programmer 125. In another embodiment, the userselects a desired performance parameter (e.g., desired degree of rateregularization, desired attack slope, desired decay slope, etc.) from acorresponding range of possible values, and device 105 automaticallyselects the appropriate combination of coefficients of filter 515 toprovide a filter setting that corresponds to the selecteduser-programmed performance parameter, as illustrated generally by Table2. Other levels of programmability or different combinations ofcoefficients may also be used. TABLE 2 Example of Automatic Selection ofAspects of Filter Setting Based on a User-Programmable PerformanceParameter. User-Programmable Performance Parameter Intrinsic Coefficienta Paced Coefficient b 1 (Less Rate 2.0 3.0   Regularization) 2 1.8 2.6 31.6 2.2 4 1.4 1.8 5 1.2 1.4 6 (More Rate 1.0 1.0   Regularization)

FILTER RATE BEHAVIOR EXAMPLE 2

FIG. 12 is a graph illustrating generally, by way of example, but not byway of limitation, one embodiment of selecting between more than oneindicated pacing interval. FIG. 12 is similar to FIG. 11 in somerespects, but FIG. 12 includes a second indicated pacing interval. Inone embodiment, the first indicated pacing interval is the VRR indicatedpacing interval, described above, and the second indicated pacinginterval is a sensor indicated pacing interval, from an accelerometer,minute ventilation, or other indication of the patient's physiologicalneed for increased cardiac output.

In one embodiment, a selected indicated pacing interval is based on theshorter of the first and second indicated pacing intervals. Stateddifferently, device 105 provides pacing pulses at the higher indicatedpacing rate. In the example illustrated in FIG. 12, first and secondbeats and the twelfth through fifteenth beats are paced at the sensorindicated rate, because it is higher than the VRR indicated rate and theintrinsic rate. The third, fourth, fifth, and ninth beats are sensedintrinsic beats that are sensed during the shorter of either of the VRRand sensor indicated pacing intervals. The sixth through eighth beatsand tenth and eleventh beats are paced at the VRR indicated rate,because it is higher than the sensor indicated rate. Also, for thesebeats, no intrinsic beats are sensed during the VRR indicated intervals.In one embodiment, the above-described equations for filter 515 operateto increase the VRR indicated rate toward the sensor-indicated rate whenthe sensor indicated rate is greater than the VRR indicated rate, asillustrated by first through third and twelfth through fifteenth beatsin FIG. 12. In an alternate embodiment, however,T_(n)=b·w·VV_(n)+(1−w)·T_(n-1, if VV) _(n) is concluded by a VRRindicated paced beat, and T_(n)=T_(n-1) if VV_(n) is concluded by asensor indicated paced beat, thereby leaving the VRR indicated rateunchanged for sensor indicated paced beats.

In this embodiment, the ranges of both the sensor indicated rate and theVRR indicated rate are limited so that they do not extend to rateshigher than the URL or to rates lower than the LRL. In one embodiment,the LRL and the URL are programmable by the user, such as by usingremote programmer 125.

In a further embodiment, the selected indicated pacing interval is basedon the shorter of the first and second indicated pacing intervals onlyif an atrial tachyarrhythmia, such as atrial fibrillation, is present.Otherwise, the second indicated pacing interval is used, as describedabove.

FILTER RATE BEHAVIOR EXAMPLE 3

FIG. 13 is a graph illustrating generally, by way of example, but not byway of limitation, another illustrative example of heart rate vs. timeaccording to a spreadsheet simulation of the behavior of theabove-described VRR algorithm. In FIG. 13, the VRR algorithm is turnedoff until time 130. Stable intrinsic lower rate behavior is modeled fortimes between 0 and 10 seconds. Erratic intrinsic ventricular rates,such as would result from atrial tachyarrhythmias including atrialfibrillation, are modeled during times between 10 seconds and 130seconds. At time 130 seconds, the VRR algorithm is turned on. While someerratic intrinsic beats are subsequently observed, the VRR algorithmprovides pacing that is expected to substantially stabilize the heartrate, as illustrated in FIG. 13. The VRR indicated pacing rate graduallydecreases until intrinsic beats are sensed, which results in a slightincrease in the VRR indicated pacing rate. Thus, the VRR algorithmfavors the patient's intrinsic heart rate when it is stable, and pacesat the VRR indicated heart rate when the patient's intrinsic heart rateis unstable. It is noted that FIG. 13 does not represent clinical data,but rather provides a simulation model that illustrates one example ofhow the VRR algorithm is expected to operate.

FILTER EXAMPLE 4

In one embodiment, filter 515 includes variable coefficients such as,for example, coefficients that are a function of heart rate (or itscorresponding time interval). In one example, operation of the filter515 is described by T_(n)=a·w·VV_(n)+(1−w)·T_(n-1), if VV_(n) isconcluded by an intrinsic beat, otherwise is described byT_(n)=b·w·VV_(n)+(1−w)·T_(n-1), if VV_(n) is concluded by a paced beat,where at least one of a and b are linear, piecewise linear, or nonlinearfunctions of one or more previous V-V intervals such as, for example,the most recent V-V interval, VV_(n).

FIG. 14 is a graph illustrating generally, by way of example, but not byway of limitation, one embodiment of using at least one of coefficientsa and b as a function of one or more previous V-V intervals such as, forexample, the most recent V-V interval VV_(n). In one such example, a isless than 1.0 when VV_(n) is at or near the lower rate limit (e.g., 1000millisecond interval or 60 beats/minute), and a is greater than 1.0 whenVV_(n) is at or near the upper rate limit (e.g., 500 millisecondinterval or 120 beats/minute). For a constant b, using a smaller valueof a at lower rates will increase the pacing rate more quickly forsensed events; using a larger value of a at higher rates increases thepacing rate more slowly for sensed events. In another example, b isclose to 1.0 when VV_(n) is at or near the lower rate limit, and b isgreater than 1.0 when VV_(n) is at or near the upper rate limit. For aconstant a, using a smaller value of b at lower rates will decrease thepacing rate more slowly for paced events; using a larger value of b athigher rates decreases the pacing rate more quickly for paced events.

CONCLUSION

It is to be understood that the above description is intended to beillustrative, and not restrictive. Many other embodiments will beapparent to those of skill in the art upon reviewing the abovedescription. The scope of the invention should, therefore, be determinedwith reference to the appended claims, along with the full scope ofequivalents to which such claims are entitled.

1. A system comprising: a ventricular sensing circuit configured tosense ventricular beats; a controller, coupled to the ventricularsensing circuit, wherein the controller is configured to obtain V-Vintervals between the ventricular beats and to compute a first indicatedpacing interval by summing a first addend that includes a most recentV-V interval duration and a second addend that includes a storedpreviously-computed V-V interval duration; a ventricular therapycircuit, coupled to the controller, configured to provide pacing therapyusing the first indicated pacing interval; and wherein the controller isconfigured to decrease the first pacing interval following an intrinsicventricular beat.
 2. The system of claim 1, wherein the controller isconfigured to increase the first pacing interval following a pacedventricular beat.
 3. The system of claim 1, wherein the storedpreviously-computed V-V interval duration consists of the V-V intervalduration directly preceding the most recent V-V interval duration. 4.The system of claim 1, wherein the controller is configured to weight atleast one of the first addend and the second addend.
 5. The system ofclaim 4, wherein the controller is configured to weight at least one ofthe first addend and the second addend using information from at leastone of a most recent ventricular beat of the most recent V-V intervalduration and a most recent ventricular beat of the storedpreviously-computed V-V interval duration.
 6. The system of claim 5,wherein the information from the most recent ventricular beat includeswhether the most recent ventricular beat includes an intrinsicventricular beat or a paced ventricular beat.
 7. The system of claim 1,wherein the controller is configured to compute the first indicatedpacing interval (T_(n)) according to: T_(n)=a·w·VV_(n)+(1−w)·T_(n-1), ifVV_(n) is concluded by an intrinsic beat, wherein a and w arecoefficients, otherwise T_(n) is computed according to:T_(n)=b·w·VV_(n)+(1−w)·T_(n-1), if VV_(n) is concluded by a paced beat,wherein b and w are coefficients.
 8. The system of claim 7, wherein atleast one of a, b, and w is a function of heart rate.
 9. The system ofclaim 1, including: a sensor, coupled to the controller, configured tosense a physiological signal; wherein the controller is configured tocompute a second indicated pacing interval using the physiologicalsignal; and wherein the ventricular therapy circuit is configured toprovide pacing therapy using the second indicated pacing interval. 10.The system of claim 1, wherein the controller is configured to compute afirst indicated pacing interval by summing a first addend that includesa most recent V-V interval duration with a second addend that includes astored previously-computed value of the first indicated pacing interval.11. The system of claim 10, wherein the stored previously-computed valueof the first indicated pacing interval consists of the directlypreceding first indicated pacing interval.
 12. A system comprising:means for sensing ventricular beats; means for obtaining V-V intervalsbetween the ventricular beats; means for computing a first indicatedpacing interval by summing a first addend with a second addend, whereinthe first addend includes a most recent V-V interval duration and thesecond addend includes a stored previously-computed V-V intervalduration; means for providing pacing therapy using the first indicatedpacing interval; and means for decreasing the first pacing intervalfollowing an intrinsic ventricular beat.
 13. A method comprising:sensing ventricular beats; obtaining V-V intervals between theventricular beats; computing a first indicated pacing interval bysumming a first addend with a second addend, wherein the first addendincludes a most recent V-V interval duration and the second addendincludes a stored previously-computed V-V interval duration; providingpacing therapy using the first indicated pacing interval; and decreasingthe first pacing interval following an intrinsic ventricular beat. 14.The method of claim 13, including increasing the first pacing intervalfollowing a paced ventricular beat.
 15. The method of claim 13, whereinthe stored previously-computed V-V interval duration consists of the V-Vinterval duration directly preceding the most recent V-V intervalduration.
 16. The method of claim 13, including weighting at least oneof the first addend and the second addend.
 17. The method of claim 13,including weighting at least one of the first addend and the secondaddend using information from at least one of a most recent ventricularbeat of the most recent V-V interval duration and a most recentventricular beat of the stored previously-computed V-V intervalduration.
 18. The method of claim 17, wherein using information from themost recent ventricular beat includes using whether the most recentventricular beat includes an intrinsic ventricular beat or a pacedventricular beat.
 19. The method of claim 13, wherein computing thefirst indicated pacing interval (T,) includes computing T_(n) accordingto: T_(n)=a·w·VV_(n)+(1−w)·T_(n-1), if VV_(n) is concluded by anintrinsic beat, wherein a and w are coefficients, otherwise computingT_(n) includes computing T_(n) according to:T_(n)=b·w·VV_(n)+(1−w)·T_(n-1) if VV_(n) is concluded by a paced beat,wherein b and w are coefficients.
 20. The method of claim 19, wherein atleast one of a, b, and w is a function of heart rate.
 21. The method ofclaim 13, including: sensing a physiological signal; computing a secondindicated pacing interval using the physiological signal; and providingpacing therapy using the second indicated pacing interval.
 22. Themethod of claim 13, wherein computing a first indicated pacing intervalincludes by summing a first addend that includes a most recent V-Vinterval duration with a second addend that includes a storedpreviously-computed value of the first indicated pacing interval. 23.The method of claim 22, wherein the stored previously-computed value ofthe first indicated pacing interval consists of the directly precedingfirst indicated pacing interval.